Thin module system and method

ABSTRACT

A flexible circuit has contacts for mounting in a socket or card edge connector. The flexible circuit includes integrated circuit devices mounted on both sides of the edge connector contacts. Preferably, the flexible circuit is wrapped about an edge of a rigid substrate and presents contacts on both sides of the substrate for mounting in a socket. Multiple flexible circuits may be overlaid with the same strategy. The flexible circuit may exhibit one or two or more conductive layers, and may have changes in the layered structure or have split layers.

FIELD

The present invention relates to systems and methods for creating highdensity circuit modules.

BACKGROUND

A variety of techniques are used to make high density circuit modules.Some techniques require special circuit board designs, while othertechniques use conventional circuit boards.

Memory expansion is one of the many fields in which high density circuitboard solutions provide space-saving advantages. For example, thewell-known DIMM (Dual In-line Memory Module) board has been used foryears, in various forms, to provide memory expansion. A typical DIMMincludes a conventional PCB (Printed Circuit Board) with memory devicesand supporting digital logic devices mounted on both sides. The DIMM istypically mounted in the host computer system by inserting acontact-bearing edge of the DIMM into a card edge connector. Typically,systems that employ DIMMs provide limited space for such devices andmost memory expansion boards are somewhat limited in the memory capacitythey add to a system.

There are several known methods to improve the limited capacity of aDIMM or other circuit board. Such methods have various cost orperformance impacts. Further, many capacity increasing techniquesexacerbate profile issues and contribute to thermal managementcomplexities.

In one scheme, small circuit boards (daughter cards) are connected tothe DIMM to provide extra mounting space. The additional connection maycause, however, flawed signal integrity for the data signals passingfrom the DIMM to the daughter card. For example, signal traces betweendevices on the DIMM and devices on the daughter card may at higherspeeds add to signal dispersion while added connectors are aconsiderable reliability issue. Other problems may arise from theconnector that attaches the daughter card to the DIMM. Such flaws maycause reflections and compromise the quality of signaling waveforms andreduce the maximum speed at which the devices may operate.

Another scheme to increase circuit board capacity is multiple diepackages (MDP). This scheme increases the capacity of the memory deviceson the DIMM by including multiple semiconductor die in a single devicepackage. The additional heat generated by the multiple die typicallyrequires, however, additional cooling capabilities to operate at maximumoperating speed. Further, the MDP scheme may exhibit increased costsbecause of increased yield loss from packaging together multiple diethat are not fully pre-tested.

Yet another strategy to increase circuit board capacity is stackedpackages. This scheme increases capacity by stacking packaged integratedcircuits to create a high-density circuit module for mounting on thecircuit board. In some techniques, flexible conductors are used toselectively interconnect packaged integrated circuits. Staktek GroupL.P. has developed numerous systems for aggregating CSP (chipscalepackaged) devices in space saving topologies. The increased componentheight of some stacking techniques may alter, however, systemrequirements such as, for example, required cooling airflow or theminimum spacing around a circuit board on its host system.

Typically, the known methods raise thermal management issues. Forexample, when a conventional FBGA packaged DRAM is mounted on a DIMM,the primary thermal path is through the balls into the core of amultilayer DIMM. When, for example, a stack of devices is employed on aDIMM, the top device gets hotter when it is active versus when the lowerdevice is active, thus stacking methods in DIMM applications may presentthermal constraints.

What is needed therefore are methods and structures for providing highcapacity circuit boards in thermally efficient, reliable designs thatperform well at higher frequencies but are not too large, yet can bemade at reasonable cost with commonly available and readily managedmaterials.

SUMMARY

A flexible circuit has contacts for mounting in a socket or card edgeconnector. Preferred embodiments of the present invention can be used toprovide an increased surface area circuit board module.

In one preferred embodiment, a flexible circuit is populated on bothsides with integrated circuits and wrapped about an edge of a rigidsubstrate. The flexible circuit presents contacts for mounting theassembly in a socket. Multiple flex circuits may be overlaid with thesame scheme. The flex circuit may aligned using tooling holes in theflex circuit and substrate. The flexible circuit may exhibit one or twoor more conductive layers, and may have changes in the layered structureor have split layers.

In another preferred embodiment, the invention provides a method ofassembling a circuit module including mounting ICs on both sides of aflexible circuit having contacts, providing a rigid substrate, andwrapping the flexible circuit around the substrate to present contactsnear the edge of the substrate for insertion into an expansion boardslot.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a contact-bearing first side of a flex circuit devised inaccordance with a preferred embodiment of the present invention.

FIG. 2 depicts the second side of the flex circuit of FIG. 1.

FIG. 3 depicts a cross-sectional view of a module assembly devised inaccordance with a preferred embodiment of the present invention.

FIG. 4 is an enlarged view of the area marked “A” in FIG. 3.

FIG. 5 is a plan view depicting one side of a module assembly devised inaccordance with a preferred embodiment of the present invention.

FIG. 6 is an enlarged view of a portion of one preferred embodiment.

FIG. 7 depicts a cutout portion of a flex circuit and substrateaccording to one preferred embodiment.

FIG. 8 depicts another embodiment of the present invention having aclip.

FIG. 9 depicts another embodiment having a thinned portion of substrate.

FIG. 10 is a cross-sectional view of another preferred embodiment of thepresent invention.

FIG. 11 depicts another preferred embodiment having an extendedsubstrate.

FIG. 12 depicts alternate preferred embodiment having additional layersof ICs.

FIG. 13 depicts another embodiment having flex portions wrapped aroundopposing edges of a substrate.

FIG. 14 depicts yet another embodiment having a flex portion wrappedaround opposing edges of a substrate.

FIG. 15 is a cross-sectional view of another embodiment of the presentinvention.

FIG. 16 depicts an alternative embodiment of the present invention.

FIG. 17 depicts an alternative embodiment of the present inventionhaving CSPs on the external side of a flex circuit.

FIG. 18 depicts an alternative embodiment of the present inventionhaving CSPs mounted between a flex circuit and substrate.

FIG. 19 depicts an alternative embodiment of the present invention inwhich the flex circuit transits over an end of the substrate oppositethe edge connector contacts.

FIG. 20 is a preferred embodiment of the present invention similar tothat depicted in earlier FIG. 11.

FIG. 21 depicts an alternative embodiment of the present invention inwhich a connector provides selective interconnective facility betweenparts of the flex circuit on opposite lateral sides of the substrate.

FIG. 22 depicts details from the area marked “A” in FIG. 21.

FIG. 23 is an elevation view of an embodiment of an alternative circuitmodule.

FIG. 24 is a cross-sectional view of the embodiment of FIG. 23.

FIG. 25 is an elevation view of another embodiment of the alternativecircuit module of FIG. 23.

FIG. 26 is a cross-sectional view of the embodiment of FIG. 25.

FIG. 27 is an elevation view of yet another alternative circuit module.

FIG. 28 is a cross-sectional view of the alternative circuit module ofFIG. 27.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 depict opposing sides 8 and 9 of a preferred flex circuit12 (“flex”, “flex circuitry”, “flexible circuit”) used in constructing apreferred embodiment of the present invention. Flex circuit 12 ispreferably made from conductive layers supported by one or more flexiblesubstrate layers as further described with reference to later Figures.The construction of flex circuitry is known in the art. The entirety ofthe flex circuit 12 may be flexible or, as those of skill in the artwill recognize, the flexible circuit structure 12 may be made flexiblein certain areas to allow conformability to required shapes or bends,and rigid in other areas to provide rigid and planar mounting surfaces.Preferred flex circuit 12 has openings 17 for use in aligning flexcircuit 12 to substrate 14 during assembly.

ICs 18 on flexible circuit 12 are, in this embodiment, chip-scalepackaged memory devices. For purposes of this disclosure, the termchip-scale or “CSP” shall refer to integrated circuitry of any functionwith an array package providing connection to one or more die throughcontacts (often embodied as “bumps” or “balls” for example) distributedacross a major surface of the package or die. CSP does not refer toleaded devices that provide connection to an integrated circuit withinthe package through leads emergent from at least one side of theperiphery of the package such as, for example, a TSOP.

Embodiments of the present invention may be employed with leaded or CSPdevices or other devices in both packaged and unpackaged forms but wherethe term CSP is used, the above definition for CSP should be adopted.Consequently, although CSP excludes leaded devices, references to CSPare to be broadly construed to include the large variety of arraydevices (and not to be limited to memory only) and whether die-sized orother size such as BGA and micro BGA as well as flip-chip. As those ofskill will understand after appreciating this disclosure, someembodiments of the present invention may be devised to employ stacks ofICs each disposed where an IC 18 is indicated in the exemplar FIGS.

Multiple integrated circuit die may be included in a package depicted asingle IC 18. While in this embodiment memory ICs are used to provide amemory expansion board, this is not limiting and various embodiments mayinclude a variety of integrated circuits and other components. Suchvariety may include microprocessors, FPGA's, RF transceiver circuitry,digital logic, as a list of non-limiting examples, or other circuits orsystems which may benefit from a high-density circuit board capability.Circuit 19 depicted between a pair of ICs 18 may be a memory buffer orcontroller.

FIG. 1 depicts a top or outer side 8 of flex circuit 12 having ICs 18mounted in two rows IC_(R1) and IC_(R2). Contact arrays are disposedbeneath ICs 18 and circuit 19 to provide conductive pads forinterconnection to the ICs. An exemplar contact array 11A is shown as isexemplar IC 18 to be mounted at contact array 11A as depicted. Thecontact arrays 11A that correspond to an IC row (e.g., IC_(R1)) may beconsidered a contact array set. Between the rows IC_(R1) and IC_(R2) ofICs 18, flex circuit 12 has two rows (C_(R1) and C_(R2)) of modulecontacts 20. When flex circuit 12 is folded as depicted in later FIGS. 3and 4, side 8 depicted in FIG. 1 is presented at the outside of module10. The opposing side 9 of flex circuit 12 (FIG. 2) is on the inside inthe folded configurations of FIGS. 3 and 4. The depiction of FIG. 1shows two pluralities of ICs 18 along side 8 of flex circuit 12, thepluralities or sets of ICs being referenced in FIG. 1 as IC_(R1) andIC_(R2). Other embodiments may have other numbers of rows and there maybe only one such row. FIG. 2 depicts another two pluralities of ICs 18along side 9 of flex circuit 12 referenced as IC_(R3) and IC_(R4).Various discrete components such as termination resistors, bypasscapacitors, and bias resistors may also be mounted on each of sides 8and 9 of flex 12. Such discrete components are not shown to simplify thedrawing. Flex circuit 12 may also depicted with reference to itsperimeter edges, two of which are typically long (PE_(long1) andPE_(long2)) and two of which are typically shorter (PE_(short1) andPE_(short2)). Other embodiments may employ flex circuits 12 that are notrectangular in shape and may be square in which case the perimeter edgeswould be of equal size or other convenient shape to adapt tomanufacturing particulars. However, rectangular shapes for flex circuit12 assist in providing a low profile for a preferred module devised withuse of flex circuit 12.

FIG. 1 depicts exemplar conductive traces 21 connecting rows C_(R1) andC_(R2) of module contacts 20 to ICs 18. Only a few exemplar traces areshown to simplify the drawing. Traces 21 may also connect to vias thatmay transit to other conductive layers of flex 12 in certain embodimentshaving more than one conductive layer. Shown is a via 23 connecting asignal trace 23 from circuit 19 to a trace 25 disposed on anotherconductive layer of flex 12 as illustrated by the dotted line of trace25. In a preferred embodiment, vias connect ICs 18 on side 9 of flex 12(FIG. 2) to module contacts 20. Traces 21 and 25 may make otherconnections between the ICs on either side of flex 12 and may traversethe rows of module contacts 20 to interconnect ICs. Together the varioustraces and vias make interconnections needed to convey data and controlsignals to the various ICs. Those of skill will understand that thepresent invention may be implemented with only a single row of modulecontacts 20 and may, in other embodiments be implemented as a modulebearing ICs on only one side.

FIG. 3 is a cross section view of a module assembly 10 devised inaccordance with a preferred embodiment of the present invention. Moduleassembly 10 is populated with ICs 18 having top surfaces 18 _(T) andbottom surfaces 18 _(B). Substrate 14 has a first and a second perimeteredges 16A and 16B appearing in the depiction of FIG. 3 as ends.Substrate 14 typically has first and second lateral sides S₁ and S₂.Flex 12 is wrapped about perimeter edge 16A of substrate 14, which inthe depicted embodiment, provides the basic shape of a common DIMM boardform factor such as that defined by JEDEC standard MO-256. Preferably,at least a portion 24 of the pocket of flex 12 formed by the wrappingabout the substrate is laminated or otherwise connected to substrate 14on both sides of substrate 14. Portion 24 may vary in length dependingon factors such as, for example, the height of ICs 18, the thickness ofsubstrate 14, the length of module contacts 20, and the size and designof the card edge connector or socket into which module assembly 10 isadapted to be mounted. Above portion 24 is depicted flex leveltransition 26 of flex circuit 12. The space between flex leveltransition 26 and substrate 14 may be filled with a conformal or heatconductive underfill, or may be left unfilled. Flex level transition 26is a bend formed in a manner devised to allow flex circuit 12 to provideconductive connection from a plane at the level of flex circuit portion24 to a plane at the level of flex circuit portion 28. The offsetbetween the two planes is, in this embodiment, the height of a single IC18 added to that of adhesive 30 (FIG. 6). Adhesive 30 in a preferredembodiment is a thermally conductive material to take advantage of theheat dissipation characteristics that may be provided by use of anappropriately selected substrate 14 comprised, for example, of a metalsuch as aluminum.

The inner pair of the four depicted ICs 18 are preferably attached tosubstrate 14 with a heat conductive adhesive 30. While in thisembodiment, the four depicted ICs are attached to flex circuit 12 inopposing pairs, this is not limiting and more ICs may be connected inother arrangements such as, for example, staggered or offsetarrangements. Further, while only CSP packaged ICs are shown, other ICsand components may be attached. In a preferred embodiment, ICs 18 willbe memory CSPs and various discrete components such as, for example,resistors and capacitors will also be mounted on flex circuit portion28. To simplify the drawing, the discrete components are not shown.Further, ICs and other components may be mounted to flex circuit portion24.

In this embodiment, flex circuit 12 has module contacts 20 positioned ina manner devised to fit in a circuit board card edge connector or socketand connect to corresponding contacts in the connector (not shown).While module contacts 20 are shown protruding from the surface of flexcircuit 12, this is not limiting and other embodiments may have flushcontacts or contacts below the surface level of flex 12. Substrate 14supports module contacts 20 from behind flex circuit 12 in a mannerdevised to provide the mechanical form required for insertion into asocket. While the depicted substrate 14 has uniform thickness, this isnot limiting and in other embodiments the thickness or surface ofsubstrate 14 in the vicinity of perimeter edge 16A may differ from thatin the vicinity of perimeter edge 16B. Non-limiting examples of suchpossible variations are found in FIGS. 9 and 10. Substrate 14 in thedepicted embodiment is preferably made of a metal such as aluminum orcopper, as non-limiting examples, or where thermal management is less ofan issue, materials such as FR4 (flame retardant type 4) epoxy laminate,PTFE (poly-tetra-fluoro-ethylene) or plastic. In another embodiment,advantageous features from multiple technologies may be combined withuse of FR4 having a layer of copper on both sides to provide a substrate14 devised from familiar materials which may provide heat conduction ora ground plane.

One advantageous methodology for efficiently assembling a circuit module10 such as described and depicted herein is as follows. In a preferredmethod of assembling a preferred module assembly 10, flex circuit 12 isplaced flat and both sides populated according to circuit board assemblytechniques known in the art. Flex circuit 12 is then folded about end16A of substrate 14. Next, tooling holes 17 may be used to align flex 12to substrate 14. Flex 12 may be laminated or otherwise attached tosubstrate 14 at portions 24. Further, top surfaces 18 _(T) of ICs 18 maybe attached to substrate 14 in a manner devised to provide mechanicalintegrity or thermal conduction.

FIG. 4 is an enlarged view of the area marked ‘A’ in FIG. 3. Edge 16A ofsubstrate 14 is shaped like a male side edge of an edge card connector.While a particular oval-like configuration is shown, edge 16A may takeon other shapes devised to mate with various connectors or sockets. Theform and function of various edge card connectors are well know in theart. Flex 12 is wrapped around edge 16A of substrate 14 and may belaminated or adhesively connected to substrate 14 with adhesive 30. Thedepicted adhesive 30 and flex 12 may vary in thickness and are not drawnto scale to simplify the drawing. The depicted substrate 14 has athickness such that when assembled with the flex 12 and adhesive 30 thethickness measured between module contacts 20 falls in the rangespecified for the mating connector. In some other embodiments, flexcircuit 12 may be wrapped about perimeter edge 16B or both perimeteredges 16A and 16B of substrate 14.

FIG. 5 depicts a plan view of module assembly 10 devised in accordancewith a preferred embodiment of the present invention. Those of skillwill recognize that module assembly 10 may replace more traditionalDIMMs employed in a large variety of systems. Module assembly 10 hasflex circuit 12 wrapped about an edge 16 of substrate 14. ICs 18 aremounted to flex circuit 12 along the depicted side as described withreference to earlier Figs. Module contacts 20 are presented near edge 22of module assembly 10 for connection to a card edge connector or socket.

FIG. 6 is an enlarged view of a portion of one preferred embodimentshowing lower IC 18 ₁ and upper IC 18 ₂. Flex 12 has flex leveltransition 26 bending from flex circuit portion 24 to flex circuitportion 28. Flex level transition 26 has, in this embodiment, a flexiblebase layer 62 and a conductive layer 66. In this embodiment, conductivelayer 66 contains conductive traces connecting module contacts 20 onflex portion 24 to BGA contacts 63 on ICs 18 ₁ and 18 ₂. Flex portion 24has two layers, but this is not limiting and other embodiments may haveother numbers of layers. The number of layers may be devised in a mannerto achieve the bend radius required to bend around edge 16A (FIG. 4) or16B, for example. The number of layers in any particular portion of flexcircuit 12 may also be devised to achieve the necessary connectiondensity given a particular minimum trace width associated with the flexcircuit technology used.

In this embodiment, there are three layers at flex portion 28 betweenthe two depicted ICs 18 ₁ and 18 ₂. Conductive layers 64 and 66 expressconductive traces that connect to the ICs and may further connect toother discrete components (not shown). Preferably, the conductive layersare metal such as, for example, copper or alloy 110. Vias such as theexemplar vias 23 connect the two conductive layers 64 and 66 and enableconnection between conductive layer 64 and module contacts 20. In thispreferred embodiment having a three-layer flex portion 28, the twoconductive layers 64 and 66 may be devised in a manner so that one ofthem has substantial area employed as a ground plane. The other layermay employ substantial area as a voltage reference plane. The use ofplural conductive layers provides advantages and the creation of adistributed capacitance intended to reduce noise or bounce effects thatcan, particularly at higher frequencies, degrade signal integrity, asthose of skill in the art will recognize. If more than two conductivelayers are employed, additional conductive layers may be added withinsulating layers separating conductive layers. Flex circuit portions 28and 24 may in some embodiments be rigid portions (rigid-flex).Construction of rigid-flex circuitry is known in the art.

With the construction of an embodiment such as that shown in FIG. 6,thermal energy will be urged to move from IC 18 ₁ into substrate 14 asexemplified by thermal vector T₁ and from IC 18 ₁ into IC 18 ₂ when IC18 ₁ is active. Thus, IC 18 ₂ assists in cooling IC 18 ₁, consequentlyproviding improved thermal dissipation as the heat traveling from IC 18₁ travels more readily through flex circuit 12 and the contacts 63 thanthrough less thermally conductive materials such as PCB materials.Further, flex circuit 12 may be particularly devised to operate as aheat spreader or sink adding to the thermal conduction out of ICs 18 ₁and 18 ₂.

FIG. 7 depicts a cutout portion of a flex circuit 12 and substrate 14according to one preferred embodiment. Flex 12 has openings 17 for usein aligning flex 12 to substrate 14 during assembly. Such alignment maybe accomplished by inserting a tooling piece along the path depicted bydotted line 76 through opening 17 on flex 12 and through correspondingopening 17 on substrate 14. Multiple openings 17 which may function astooling holes may appear in various places. Further, the alignmentbetween flex circuit 12 and substrate 14 may also be implemented, forexample, with an opening and protrusion combination such as a slot andtab arrangement or a hole and pin arrangement, for example. Those ofskill will be able to readily adapt the teachings of this disclosure todevise corresponding opening and protrusion arrangements for alignmentof flex and substrate in accordance with the present invention.

Depicted are indents 72 which may be required by certain card edgeconnectors. Similar indents will typically appear along edge 16 (FIG. 5)of substrate 14 and may require corresponding holes or indents in flexcircuit 12 to match mechanical features on certain card edge connectors.

FIG. 8 depicts another embodiment having a clip. In this embodiment,clip 82 is depicted clipped around ICs 18. Clip 82 is preferably made ofmetal or other heat conducting material. Preferably, clip 82 has trough84 devised to mate with the end of substrate 14. The attachment mayfurther be accomplished with adhesive between clip 82 and substrate 14or ICs 18.

FIG. 9 depicts another embodiment having a thinned portion of substrate14. In this embodiment, substrate 14 has a first thickness 1 toward edge16A devised to provide support for an edge and surrounding area ofmodule assembly 10 as may be needed for connection to a card edgeconnector. Above the portion of substrate 14 with thickness 1 is aportion 92 having thickness 2. The narrower width of portion 92 isdevised to narrow the total width of module assembly 10 and may providefor enhanced cooling airflow or more dense spacing of module assemblies10 in their operating environment.

FIG. 10 is a cross-sectional view of another preferred embodiment. Thedepiction is facing down. Substrate 14 is selectively thinned at portion102 under device 104. Depicted device 104 has an exposed die 106 mountedon a substrate. Other embodiments may have otherwise packaged or mountedintegrated circuits or other devices with heights greater than thetypical IC 18. ICs 18 are in preferred embodiments memory CSPs allhaving similar heights. In this embodiment, device 104 is taller thanthe other ICs 18 populating the flex 12. Thinned portion 102 ofsubstrate 14 underneath device 104 accommodates the extra height so thatflex 12 remains planer and the upper surface of device 104 contactssubstrate 14. Substrate 14 may be manufactured for this or other similarembodiments with a variety of method such as, for example, by beingmilled with a CNC (computer numerical controlled) machine, or beingextruded, for example. This and similar embodiments may be employed toadvantage to provide advantageous heat performance when device 104 is aFB-DIMM advanced memory buffer (AMB). Device 104 is preferably attachedto substrate 14 with heat conductive adhesive.

FIG. 11 depicts another embodiment having an extended substrate 14.Depicted extension 112 of substrate 14 extends beyond the top of flex12. Extension 112 is shaped to provide additional surface area forconvective cooling. Such shape may be achieved by methods such as, forexample, milling or extrusion, which are both known in the art.Preferably, extruded aluminum is used for substrate 14 in this andsimilar embodiments.

FIG. 12 depicts another embodiment of the invention having additionallayers of ICs 18. In this embodiment, four flex level transitions 26connect to four mounting portions 28. Each mounting portion 28 has ICs18 on both sides. Flex circuitry 12 may be provided in thisconfiguration by, for example, having a split flex with layersinterconnected with vias at portion 24 of flex 12. Further, two flexcircuits may be used and interconnected by pad to pad contacts orinter-flex contacts.

FIG. 13 depicts another embodiment having flex portions wrapped aroundopposing edges of substrate 14. Flex circuit 12 has connecting portion132 wrapped around form portion 134 of substrate 14. Form portion 134 isa type of perimeter edge 16B shaped to provide a larger surface fortransit of the flex circuit. In a preferred methodology for assemblingthis embodiment, the depicted ICs 18 are first mounted to flex circuit12. Flex portion 26 associated with IC 18 a is placed in positionrelative to the substrate. Flex circuit 12 is then wrapped around edge16 of substrate 14 a first time. Appropriate adhesive lamination orother techniques are used to attach flex 12 and ICs 18 a and 18 b tosubstrate 14. Connecting portion 132 of flex circuit 12 is wrappedaround form portion 134. Adhesive may be used to make back-to-backconnections between the depicted ICs 18. Lamination or other adhesive orbonding techniques may be used to attach the two layers of flex 12 toeach other at flex portions 24. Further, the two layers of flexcircuitry 12 wrapped around edge 16A may interconnected with by pad topad contacts or inter-flex contacts. Flex 12 is wrapped again aroundedge 16A, putting IC 18 c into position. IC 18 d is positionedback-to-back with IC 18 e and attached.

FIG. 14 depicts another embodiment having a flex portion wrapped aroundopposing edges of substrate 14. Flex circuit 12 has connecting portion132 wrapped around form portion 134 of substrate 14. Connecting portion132 preferably has more than one conductive layer, and may have three orfour or more conductive layers. Such layers may be beneficial to routesignals for applications such as, for example, a FB-DIMM (fully-bufferedDIMM) which may have less DIMM input/output signals than a registeredDIMM, but may have more interconnect traces required among devices onthe DIMM, such as, for example, the C/A copy A and C/A copy B(command/address) signals produced by an FB-DIMM advanced memory buffer(AMB). Flex 12 terminates at end 136, which may be at the level of flexportion 28 or may extend to the level of portion 24 and be attached tosubstrate 14. While two sets of module contacts 20 are shown, otherembodiments may have only one set and may not have flex 12 wrappedaround edge 16A of substrate 14.

FIG. 15 depicts a cross-sectional view of another alternative embodimentof the present invention. Flex circuit 12 exhibits contacts 20 proximalto opposing edges 192. Connecting portion 132 of flex circuit 12 iswrapped about form portion 134 of substrate 14. Contacts 20 are, in thisembodiment, arranged proximal to opposite edges 192 of flex circuit 12.In a preferred methodology for assembling this embodiment, the depictedICs 18 are first mounted to flex circuit 12. Flex circuit 12 is wrappedabout form portion 134 of substrate 14 and preferably aligned tosubstrate 14 with tooling holes. Portion 24 of flex circuit 12 ispreferably laminated to substrate 14.

FIG. 16 depicts an alternative embodiment of the present invention.

FIG. 17 depicts an alternative embodiment of the present inventionhaving CSPs on the external side of a flex circuit.

FIG. 18 depicts an alternative embodiment of the present inventionhaving CSPs mounted between a flex circuit and substrate.

FIG. 19 depicts an alternative embodiment of the present invention inwhich the flex circuit transits over an end of the substrate oppositethe module contacts.

FIG. 20 is a preferred embodiment of the present invention similar tothat depicted in earlier FIG. 11.

FIGS. 21 and 22 depict an alternative embodiment of the presentinvention that employs a connector 200 to provide selectiveinterconnection between portions 202A and 202B of flex circuit 12associated respectively with lateral sides S₁ and S₂ of substrate 14.The depicted connector 200 has first part 200B and 200A thatinterconnect in cavity 204 of flex circuit 12. One example of connector200 is a 500024/50027 Molex connector but a variety of differentconnectors may be employed in embodiments of the invention. The depictedconnector 200 is disposed in substrate cavity and typically will have afirst part 200A and a second part 200B.

FIGS. 23 and 24 depict an alternative circuit module. In the embodimentshown in FIGS. 23 and 24 flex circuit 12 is a rigid flex. FIG. 23 is anelevation view. FIG. 24 is a cross-sectional view. As shown, flexcircuit 12 has two rigid portions 13 connected by a bend 31 at theflexible region. Imposing bend 31 in flex circuit 12 creates anopen-ended pocket 32 into which may be at least partially inserted asupport or substrate 14 as shown in earlier FIGS. and/or a heat spreadersuch as heat spreader 152 shown in FIG. 24.

Both rigid portions 13 of flex circuit 12 have ICs 18 mounted onopposing sides. Heat spreader 152 shown between rows of ICs 18, may beattached to the upper major surface of one or both of the depicted ICs18. Heat spreader 152 is preferably copper or other heat conductivemetal or metal alloy. Contacts 20 are presented along the sides of rigidportion 13 proximal to edge 16. Contacts 20 and edge 16 are sized andarranged for insertion into a card edge connector or socket.

FIGS. 25 and 26 depict another alternative circuit module. FIG. 27 is anelevation view. FIG. 28 is a cross-sectional view. In this embodiment,substrate 14 is a circuit board preferably made of FR4 having etchedcopper layers. ICs 18 are mounted along substrate 14. Additional ICs 18are mounted along flex circuit 12. Flex circuit 12 if folded over thetop edge of substrate 12 to interconnect ICs 18 on flex circuit 12. Thedepicted adjacent ICs in FIG. 26 may be attached adhesively back-to-backand may be provided with a heat spreader 152 (FIG. 24) between them.Flex level transitions 26 bend from flex portion 28 to flex portion 24.Flex portion 24, in this embodiment, has contacts for electricalconnection to substrate 14. Substrate 14 has contacts 20 for connectionto a card edge connector or socket.

FIGS. 27 and 28 depict another alternative circuit module. FIG. 28 is anelevation view. FIG. 26 is a cross-sectional view. Flex circuit 12 isbent lengthwise about substrate 14 at bend 204. At bend 202, flexcircuit 12 is bent back over heat spreader 152. Preferably, the uppermajor surfaces of the ICs 18 adjacent to substrate 14 are attached tosubstrate 14. Flex level transitions 26 _(A) and 26 _(B) bend to alignportion 24 of flex circuit 12 for attachment to substrate 14. Flextransition 26 _(A) passes through slot 121 formed in substrate 14. Inthis alternative embodiment, substrate 14 is shaped in a manner devisedto center contacts 20 in the cross-section. Some contacts 20 aredepicted on substrate 14. In this embodiment, substrate 14 is preferablya circuit board made of FR4. Portions 24 of flex circuit 12 may havecontact pads for electrical connection to corresponding contact pads onsubstrate 14. In other embodiments, flex circuit 12 may be folded aboutthe edge of substrate 14 or contacts 20 may appear on only one side ofmodule 10.

Although the present invention has been described in detail, it will beapparent to those skilled in the art that many embodiments taking avariety of specific forms and reflecting changes, substitutions andalterations can be made without departing from the spirit and scope ofthe invention. The described embodiments illustrate the scope of theclaims but do not restrict the scope of the claims.

1. A memory expansion board comprising: (a) a rigid substrate having twoopposing lateral sides and an edge; (b) a flex circuit wrapped about theedge of the rigid substrate, the flex circuit having a first side and asecond side, a portion of the flex circuit attached to at least one ofthe lateral sides of the rigid substrate, the flex circuit having pluralcontacts adapted for connection to a circuit board socket, the pluralcontacts being disposed near the edge of the rigid substrate on theoutside side of the flex circuit; (c) plural memory CSPs mounted on thefirst side and second side of the flex circuit.
 2. The memory expansionboard of claim 1 in which the plural CSPs each have a top surface andone or more of the top surfaces of the plural CSPs is attached to therigid substrate.
 3. The memory expansion board of claim 1 in which therigid substrate is made of a conductive material.
 4. The memoryexpansion board of claim 1 in which the rigid substrate is made of athermally conductive material.
 5. The memory expansion board of claim 1in which the rigid substrate has an extension.
 6. The memory expansionboard of claim 1 further comprising at least one alignment opening ofthe flex circuit matching at least one alignment opening of the rigidsubstrate.
 7. The memory expansion board of claim 1 further comprisingat least one alignment opening of the flex circuit matching at least onealignment protrusion of the rigid substrate.
 8. The memory expansionboard of claim 1 further comprising at least one alignment tab of therigid substrate.
 9. A circuit module comprising: a substrate having afirst and a second lateral side and a first perimeter edge and a secondperimeter edge; a flex circuit having a first side and a second side,the first side having expansion board contacts adapted for connection toan expansion board slot and having a set of contact arrays, the flexcircuit being wrapped about the first perimeter edge of the substrate toplace the expansion board contacts of the first side closer to the firstperimeter edge of the substrate than is disposed the set of contactarrays and to place the second side of the flex circuit closer to thelateral sides of the substrate than is disposed the first side of theflex circuit.
 10. A circuit module comprising: a substrate having afirst and a second lateral side and a first perimeter edge and a secondperimeter edge; a flex circuit having a first side and a second side,the first side having expansion board contacts adapted for connection toan expansion board slot and having a set of contact arrays, the flexcircuit being wrapped about the first perimeter edge of the substrate toplace the expansion board contacts of the first side closer to thesecond perimeter edge of the substrate than is disposed the set ofcontact arrays and to place the second side of the flex circuit closerto the lateral sides of the substrate than is disposed the first side ofthe flex circuit.
 11. A circuit module comprising a flex circuit havinga first side having contacts adapted for connection to a socket, asecond side, and being imposed with a bend to form an open-ended pockethaving an inward side and an outward side and being open at one end andclosed at the other end of the pocket, the first side of the flexcircuit being on the outward side of the pocket and the second side ofthe flex circuit being on the inward side of the pocket.
 12. The circuitmodule of claim 11 further comprising a rigid interposer disposed atleast partially in the open-ended pocket of the flex circuit.
 13. Thecircuit module of claim 12 in which the rigid interposer is made of aconductive material.
 14. The circuit module of claim 11 furthercomprising a support member disposed at least partially in theopen-ended pocket of the flex circuit.
 15. The circuit module of claim11 further comprising a heat-conducting member disposed at leastpartially in the open-ended pocket of the flex circuit.
 16. The circuitmodule of claim 12, in which the rigid interposer has a necked narrowportion disposed adjacent to the closed end of the pocket.
 17. Thecircuit module 11 in which the flex circuit has two end portions, eachend portion having a plurality of memory CSPs mounted on the first andsecond sides of the flex circuit.
 18. A circuit module comprising: (a) aflex circuit having an inner side and an outer side; (b) plural CSPsmounted along the inner side and the outer side of the flex circuit; (c)a bend in the flex circuit between first and second portions of the flexcircuit, the first and second portions each having contacts arrangedalong their outer side, the contacts for mounting the circuit module ina card edge connector, the bend separating a first set of the pluralCSPs from a second set of the plural CSPs; (d) a support structure aboutwhich the flex circuit transits through the bend.
 19. The circuit moduleof claim 18 in which at least one of the CSPs on the inner side of theflex circuit is adhesively connected to the support structure.
 20. Thecircuit module of claim 18 in which at least one of the CSPs on theinner side of the flex circuit is thermally connected to the supportstructure.
 21. The circuit module of claim 18 in which a portion of theflex circuit is laminated to the support structure.
 22. The circuitmodule of claim 18 in which the bend in the flex circuit creates anopen-ended pocket having a closed end and an open end and the supportstructure exhibits a first portion having a first thickness at theclosed end of the pocket and presents a second portion having a secondthickness at the open end of the pocket in the flex circuit.
 23. Thecircuit module of claim 18 in which the support structure has a firstportion and a second portion, the first portion being thinner than thesecond portion.
 24. A method for devising a circuit module comprisingthe steps of: providing a flex circuit having first and second sides andfirst and second long perimeter edges and first and second shortperimeter edges with a set of module contacts along the first side andfirst and second pluralities of CSPs disposed laterally about the set ofmodule contacts to place the first plurality of CSPs nearer the firstlong perimeter edge of the flex circuit than is disposed the set ofmodule contacts and the second plurality of CSPs nearer the second longperimeter edge of the flex circuit than is disposed the set of modulecontacts; a substrate having first and second lateral sides and a firstlong perimeter edge and a second long perimeter edge; wrapping the flexcircuit about the substrate to dispose the second side of the flexcircuit closer to the first and second lateral sides of the substratethan is disposed the first side of the flex circuit and to dispose theset of module contacts nearer the first long perimeter edge of thesubstrate than the second long perimeter edge of the substrate and toplace the first plurality of CSPs closer to the first lateral side ofthe substrate than is disposed the second plurality of CSPs.
 25. Themethod of claim 24 in which the provided flex circuit has third andfourth pluralities of CSPs.
 26. The method of claim 24 in which the CSPsare each stacked modules composed of two or more individual CSPs.
 27. Amethod for providing increased memory capacity for a computer systemcomprising the steps of: providing a circuit module in accordance withclaim 1 and inserting said module into an expansion slot.
 28. A methodfor providing increased memory capacity for a computing systemcomprising the steps of: providing a circuit module devised inaccordance with claim 24 and inserting said module into an expansionslot on a motherboard.
 28. A method of assembling a circuit modulecomprising the steps: providing a flex circuit having a first side and asecond side, the first side having a plurality of pads for mountingcomponents and a plurality of contacts for insertion in an expansionboard slot; the second side having a plurality of pads for mountingcomponents; mounting plural CSPs along the first side of the flexcircuit; mounting plural discrete components along the first side of theflex circuit; mounting plural CSPs along the second side of the flexcircuit; mounting plural discrete components along the second side ofthe flex circuit; providing a rigid substrate having first and secondmajor surfaces and an edge; and wrapping the flex circuit about the edgeof the rigid substrate, with the first side facing outward, such that afirst set of the plurality of contacts are disposed proximal to the edgeof the rigid substrate.
 29. The method of claim 28 in which the step ofwrapping the flex circuit further includes wrapping such that a secondset of the plurality of contacts are disposed proximal to the edge ofthe rigid substrate.
 30. The method of claim 28 further including thestep of attaching at least one of the plural CSPs along the second sideof the flex circuit to the rigid substrate.
 31. The method of claim 28further including the step of thermally connecting at least one of theplural CSPs along the second side of the flex circuit to the rigidsubstrate.
 32. The method of claim 28 further including the step ofattaching a heat radiating clip to selected ones of the plural CSPs. 33.The method of claim 28 further including the step of attaching a heatradiating element to at least one of the CSPs along the first side ofthe flex circuit.
 34. The method of claim 28 further including the stepof inserting the plurality of contacts at least partially into anexpansion board slot for connection to an operating environment.
 35. Amethod of assembling a circuit module comprising the steps: providing aflex circuit having a first side and a second side and a plurality ofcontacts along the first side for insertion in an expansion board slot;mounting at least first and second CSPs along the first side of the flexcircuit; providing a rigid substrate having first and second major sidesand an edge; wrapping the flex circuit about the rigid substrate todispose the first of the at least first and second CSPs closer to thefirst major side of the rigid substrate than the second major side ofthe substrate and dispose the second of the at least first and secondCSPs closer to the second major side of the rigid substrate than thefirst major side of the substrate and attaching the flex circuit to therigid substrate such that the plural contacts are presented proximal tothe edge of the rigid substrate for insertion into the expansion boardslot.
 36. The method of claim 35 in which the step of attaching the flexcircuit to the rigid substrate comprises lamination.
 37. The method ofclaim 35 further comprising mounting third and fourth CSPs along thesecond side of the flex circuit.
 38. The method of claim 35 in which therigid substrate is made of heat conducting material.
 39. The method ofclaim 35 in which the rigid substrate has first portion and a secondportion, the first portion being thinner than the second portion. 40.The method of claim 35 further including the step of thinning the rigidsubstrate along the edge of the rigid substrate.
 41. The method of claim35 further including the step of aligning a tooling hole of the flexcircuit with a tooling hole of the rigid substrate.
 42. A populatedflexible circuit comprising: a flexible circuit having a first majorside and a second major side, the flexible circuit exhibiting along thefirst major side, first-side first and second sets of contact sitearrays between which is located a row of connector contacts, the secondmajor side of the flexible circuit exhibiting second-side first andsecond sets of contact site arrays which correspond to the first-sidefirst and second sets of contact site arrays, each of the first-side andsecond-side first and second sets of contact site arrays comprising atleast two surface mount arrays, the flexible circuit providingconnections between the at least two surface mount arrays of each of thefirst-side first and second sets of contact site arrays and the at leasttwo surface mount arrays of each of the second-side first and secondsets of contact site arrays; a plurality of CSPs that populate the atleast two surface mount arrays of each of the first-side first andsecond sets of contact site arrays and the at least two surface mountarrays of each of the second-side first and second sets of contact sitearrays.
 43. A circuit assembly comprising: a flexible circuit having afirst major side and a second major side, the flexible circuit having atleast one or more rows of surface mount arrays on the first major sideand two or more rows of surface mount arrays on the second major side,the flexible circuit having a arcuate bend between a selected two of thetwo or more rows of surface mount arrays on the second major side, thesecond major side facing inward to the arcuate bend, the flexiblecircuit having an end edge and connector contacts disposed proximal tothe end edge; a plurality of CSPs that populate the one or more rows ofsurface mount arrays of the first major side and the two or more rows ofsurface mount arrays of the second major side, each of the CSPs having atop major surface; a support substrate partially within the arcuatebend, the support substrate having a first side and a second side and anedge, at least one of the top major surfaces of the plurality of CSPspopulating the two or more rows of surface mount arrays of the secondmajor side being attached to the support substrate, the edge of thesupport substrate being adapted for insertion into a card edgeconnector.
 44. A circuit assembly comprising: a flexible circuit havinga first major side and a second major side, the flex circuit having twoor more rows of surface mount arrays on the second major side, theflexible circuit having an arcuate bend between a selected two of thetwo or more rows of surface mount arrays on the second major side, thesecond major side facing inward to the arcuate bend, the flexiblecircuit having an end edge and connector contacts disposed proximal tothe end edge; a plurality of CSPs that populate the two or more rows ofsurface mount arrays of the second major side, each of the CSPs having atop major surface; a support substrate partially within the arcuatebend, the support substrate having a first side and a second side and anedge, at least one of the top major surfaces of the plurality of CSPspopulating the two or more rows of surface mount arrays of the secondmajor side being attached to the support substrate, the edge of thesupport substrate being adapted for insertion into a card edgeconnector.
 45. A circuit module comprising: a rigid substrate having twoopposing lateral sides and two opposing end edges; a flexible circuitwrapped about at least one of the two opposing end edges, the flexiblecircuit having a first side and a second side each having one or morerows of contact site arrays, a portion of the flex circuit beingattached to at least one of the lateral sides of the circuit board, theflex circuit having plural contacts adapted for electrical connection toa card edge connector.
 46. The circuit module of claim 45 in which theplural contacts are on the first side of the flex circuit, and in whicha portion of the second side of the flex circuit opposite at least someof the plural contacts is laminated to the rigid substrate.
 47. Acircuit module comprising: a circuit board having two opposing lateralsides and an edge; a flex circuit wrapped around the edge of the rigidsubstrate, the flex circuit having an inner side and an outer side, theinner and outer sides each having two or more rows of contact sitearrays, a portion of the flex circuit being laminated to at least one ofthe lateral sides of the circuit board, the flex circuit having pluralcontacts adapted for electrical connection to the circuit board; aplurality of CSPs mounted to the two or more rows of contacts sitearrays of the inner side and the outer side of the flex circuit.
 48. Amethod to encourage the extraction of thermal energy from a CSP thatoperates in conjunction with at least one other CSP comprising the stepsof: providing a first CSP having a top surface and a bottom surface,there being CSP contacts along the bottom surface; providing a thermallyconductive substrate member and attaching the first CSP to the thermallyconductive substrate member; providing a flex circuit and attaching thefirst CSP to the flex circuit, the attachment being effectuatedemploying the CSP contacts of the first CSP and employing the thermallyconductive substrate member as a support for a part of the flex circuit;providing a second CSP having a bottom surface and CSP contacts andattaching the second CSP to the flex circuit, the attachment beingeffectuated employing the CSP contacts of the second CSP so that the CSPcontacts of the first CSP are separated from the CSP contacts of thesecond CSP by a part of the flex circuit.
 49. The method of claim 48further comprising a set of contacts electrically connected to the flexcircuit to provide connective facility for the first and second CSPs toan operating environment.
 50. The method of claim 48 in which thethermally conductive substrate member is comprised of a metal.
 51. Themethod of claim 50 in which the thermally conductive substrate member iscomprised of aluminum.
 52. The method of claim 50 in which the thermallyconductive substrate member is comprised of a radiative portion havingfins.
 53. The method of claim 48 in which the thermally conductivesubstrate member is comprised of FR4 and a metallic layer.
 54. Themethod of claim 48 in which the attachment of the first CSP to thethermally conductive substrate member is by the top surface of the firstCSP.
 55. A circuit module to encourage the extraction of thermal energyfrom a CSP that operates in conjunction with at least one other CSPcomprising: a first CSP having a top surface and a bottom surface andCSP contacts, the CSP contacts being along the bottom surface; athermally conductive substrate member attached to the first CSP; a flexcircuit attached to the first CSP, the attachment being effectuatedemploying the CSP contacts of the first CSP, the thermally conductivesubstrate member being a support for a part of the flex circuit; asecond CSP attached the flex circuit, the attachment being effectuatedemploying the CSP contacts of the second CSP so that the CSP contacts ofthe first CSP are separated from the CSP contacts of the second CSP byat least a part of the flex circuit.
 56. A circuit module comprising: asubstrate having first and second lateral sides and a cavity; flexcircuitry having a first portion adjacent to the first lateral side ofthe substrate and a second portion adjacent to the second lateral sideof the substrate; the flex circuitry having edge card connectorcontacts; a connector having first and second parts which areselectively joinable; the first part of the connector being connected tothe first portion of the flex circuitry and the second part of theconnector being connected to the second portion of the flex circuitry,the first and second parts of the flex circuitry being joined in thecavity of the substrate.